Dear sir/madam:

I intend to do a SDM meta analysis of fMRI studies for a certain contrast: say [a>b]. For some studies I can obtain the "T-maps". I have two questions about the T-maps:
1. I presume SDM uses the unthresholded (non multiple comparisons corrected) maps?
2. For some studies I also can obtain the T-map for the reversed contrast [b>a] (deactivations): how do I enter the two image files for a given study for analysis in SDM? Shouldn't I reverse the sign of the [b>a] image?

Thank you very much in advance for your response,

Jurriaan

Dear Jurriaan,


I'm not 100% sure this is completely correct but I believe as you are using the unthresholded maps (yes to your first question) this includes all the activations and deactivations which would be included in the reversed contrast. Think of thresholding in one direction as cutting off the highest points and the reversed contrast cutting off the lowest points (deactivations). As you are using unthresholded maps you have the full range of values so the information in the reverse contrast map is redundant. This is not the case for coordinates which just represent these extreme high and low values so should be entered for both directions of the contrast with the reverse contrast deactivations having negative t values.


Hope this is correct and helpful.


Jo

Thanks for your quick response Jo. I think you are quite right about your answer regarding deactivations: in the unthresholded map for [a > b] deactivations should already be in there (voxels with negative T values). But thanks for thinking along with me there: I think I have enough information now to start doing the analyses (exciting)!

Thanks again and best,

Jurriaan

Just to thank Jo for the nice response!